简单工厂模式

public class Operation
    {
        private double _numberA = 0;
        private double _numberB = 0;

        public double NumberA
        {
            get { return _numberA; }
            set { _numberA = value; }
        }

        public double NumberB
        {
            get { return _numberB; }
            set { _numberB = value; }
        }

        public virtual double GetResult()
        {
            double result = 0;
            return result;
        }
    }

    class OperationAdd : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = NumberA + NumberB;

            return result;
        }
    }

    class OperationSub : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = NumberA - NumberB;

            return result;
        }
    }

    class OperationMul : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = NumberA * NumberB;

            return result;
        }
    }

    class OperationDiv : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            if (Math.Abs(0 - NumberB) < 0.000001)
            {
                throw new Exception("除数不能为0");
            }

            result = NumberA / NumberB;

            return result;
        }
    }

public class OperationFactory
    {
        public static Operation createOperate(string operate)
        {
            Operation oper = null;

            switch (operate)
            {
                case "+":
                    oper = new OperationAdd();
                    break;
                case "-":
                    oper = new OperationSub();
                    break;
                case "*":
                    oper = new OperationMul();
                    break;
                case "/":
                    oper = new OperationDiv();
                    break;
            }

            return oper;
        }
    }

转自《大话设计模式》